Add member property to Membership

[jpmckinney]

The JSON representation of Membership doesn't allow for Org-Org memberships, which is needed to model coalitions, for example. See discussion:

https://groups.google.com/d/msg/poplus/yZJfHocAGO0/qqDXbux4m6sJ

Splitting out of https://github.com/opennorth/popolo-spec/issues/41